The present invention relates generally to communication satellites, and more particularly, to a reflector configuration for communication satellites.
Communication satellites employing multiple spot beam payloads typically require either multiple reflector antennas (3 or 4 apertures) or a single reflector with a complex beamforming network for efficient transmission as well as receiving functions. The transmission function is to be referred to as a downlink and the receiving function is referred to as an uplink. Typically, multiple reflector antennas (3 or 4) for each transmit and receive frequency band are employed. The disadvantage with this approach is that more physical space on the spacecraft body is required to mount the antennas. That is, typically both the east and west sides of the spacecraft are used for the reflectors while leaving only the nadir panel for other payloads. The reflector systems are also heavier and require larger feed horns.
Another approach is a single reflector for each frequency band and the employment of a large number of feed horns with a low-level beamforming network dedicated to each reflector. Each beam is generated by an overlapping cluster of horns, typically seven, and requires an element sharing network and a beamforming network to form multiple overlapping beams. One disadvantage of this approach is that a large number of feeds, a large number of amplifiers, and complex and heavy beamforming networks are required. This increases the complexity of the spacecraft.
Another approach is using a solid reflector with a frequency selective surface (FSS) subreflector with separate feed arrays. The FSS subreflector transmits the downlink frequencies and reflects the uplink frequencies. The number of main reflectors is reduced by a factor of two relative to the first described approach, but it requires an additional frequency selective subreflector for each main reflector. One disadvantage of this approach is that complex frequency selective surface subreflectors require more area to package on a spacecraft and the increased loss associated with the FSS subreflector which impact electrical performance.
Yet another approach is described in U.S. Pat. No. 6,140,978. In the ""978 patent a frequency selective surface main reflector and dual-band feed horns are used. The ""978 patent employs one set of reflectors where each reflector has a central solid region that is reflective to both frequency bands and an outer ring that is selective to the frequencies and is reflective at downlink frequencies and non-reflective at uplink frequencies. Thus, the electrical size of the reflector is therefore different at the two bands and thus can be adjusted to achieve the same coverage on the ground. Disadvantages of this approach are that the losses associated with the reflector are increased, the increased complexity of the reflector itself, and the increased cost and the need to diplex the feed horn results in bandwidth and passive-inter-modulation issues. Although the number of reflectors is reduced by a factor of two, three or four reflectors are still required.
It would therefore be desirable to provide a simple lightweight size for an antenna reflector to reduce the overall complexity and weight of the spacecraft.
It is therefore one object of the invention to provide a simplified antenna configuration for a spacecraft.
An important aspect of this invention is the use of a single xe2x80x9chybrid reflectorxe2x80x9d with combination of fully reflective and partially reflective surfaces in order to generate multiple beams.
In one aspect of the invention, an antenna for reflecting a frequency band comprises a central portion fully reflective to the frequency band. A number of annular bands surrounding the central portion are used with partially reflective surfaces. A first annular band is disposed directly adjacent to the central portion. The first annular band is partially reflective to the frequency band.
It should be noted that the antennas may be incorporated into a satellite wherein one antenna is used for transmitting and one antenna is used for receiving all the beams in the satellite. Because of the use of a single reflector to generate all beams within a frequency band, performance degradation due to differential pointing error among multiple apertures of a conventional design is eliminated.
One advantage of the invention is that the number of reflectors is reduced which in turn reduces the complexity and size of the spacecraft. Another advantage of the invention is that because a reduced number of reflectors are used, more space is available on the exterior of the satellite for various types of payloads. Yet another advance of this invention is that it does not require complex beam forming networks to form beams.
Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.